The present invention relates to the field of data transmission systems, particularly data transmission systems suitable for use in downhole environments, such as along a drill string used in oil and gas exploration, or along the casings and other equipment used in oil and gas production.
The goal of accessing data from a drill string has been expressed for more than half a century. As exploration and drilling technology has improved, this goal has become more important in the industry for successful oil, gas, and geothermal well exploration and production. For example, to take advantage of the several advances in the design of various tools and techniques for oil and gas exploration, it would be beneficial to have real time data such as temperature, pressure, inclination, salinity, etc. Several attempts have been made to devise a successful system for accessing such drill string data. These systems can be broken down into four general categories.
The first category includes systems that record data downhole in a module that is periodically retrieved, typically when the drill string is lifted from the hole to change drill bits or the like. Examples of such systems are disclosed in the following U.S. Patents: 3,713,334; 4,661,932 and 4,660,638. Naturally, these systems have the disadvantage that the data is not available to the drill operator in real time.
A second category includes systems that use pressure impulses transmitted through the drilling fluid as a means for data communication. For example, see U.S. Pat. No. 3,713,089. The chief drawbacks to this mud pulse system are that the data rate is slow, i.e. less than 10 baud; the system is complex and expensive; the results can be inconsistent; and the range of performance can be limited. In spite of these drawbacks, it is believed that this mud pulse system is the only real time data transmission system currently in commercial use.
The third category includes systems that transmit data along an electrical conductor that is integrated by some means into the drill string. Examples of such systems are disclosed in the following U.S. Patents: 3,879,097; 4,445,734 and 4,953,636.
U.S. Pat. No. 4,445,734, incorporated herein by this reference, teaches the use of a wire secured within the pipe by a liner. The wire is connected to annular contacts at each joint. The contact mechanism includes a means for wiping the surfaces of the contacts and of exerting pressure on the contacts in order to ensure efficient signal transmission.
Because the drill string can be comprised of several hundred sections of drill pipe, it is desirable to locate the electrical system within each section of pipe and then provide for electrical connections when the sections are joined together. A decided drawback of such systems is the fact that the downhole environment is quite harsh. The drilling mud pumped through the drill string is abrasive and typically has a high salt content. In addition, the downhole environment typically involves high pressures and temperatures. Moreover, heavy grease is typically applied at the joints between pipe sections. Consequently, the reliance on an electrical contact between joined pipe sections is typically fraught with problems.
A fourth category includes systems that use a combination of electrical and magnetic principles. In particular, such systems have an electrical conductor running the length of the drill pipe and then convert the electrical signal into a corresponding magnetic field at one end. This magnetic field is passed to the adjacent drill pipe and then converted to back to an electrical signal. Examples of such systems are described below.
U.S. Pat. No. 2,379,800 to Hare describes a system with a primary transformer coil, consisting of a wire wound around a soft iron core, being installed within an annular groove at one end of the pipe and a similar, secondary transformer coil, being installed within an annular groove at the other end of the pipe. When the pipes are connected, the primary and secondary coils are brought close together. Once the signal is transmitted across the joint, it is carried along the drill pipe by a wire connected to the coil in the opposite end of the pipe. The system also included condensers, rectifiers, and amplifiers to aid the transmission of the signal from one pipe to another.
U.S. Pat. No. 2,414,719 to Cloud, discloses a serial inductive coupling system including a series capacitor in each link to tune the system to a given pass band, typically around 3 kHz. The capacitor has the undesired feature of providing a narrow bandwidth. Cloud also suggested the use of a U-shaped trough of a “magnetic member” (see reference numeral 56 in FIG. 9). The materials suggested for this magnetic member include “Armco iron, nickel alloy, and magnetic steel.” All of these materials conduct electricity. As such, it is believed that eddy currents develop in this magnetic member, thereby lowering the efficiency of the system.
U.S. Pat. No. 3,090,031 to Lord proposed an improvement to the Hare Patent to help reduce the power required in the transformer system. Lord's patent describes a circuit similar to Hare's but with the addition of a transistor and the use of mercury-type penlight batteries as a power source at each joint. As an alternative power source, he proposed the use of chemical additives to the drilling fluid that could provide power to the transformers by electrolytic action.
U.S. Pat. No. 4,788,544 to Howard describes a system that utilized a Hall Effect sensor as a means to bridge the drill pipe joint. In this system, an electromagnetic field generating coil having a ferrite core is employed to transmit data signals across the joint. The magnetic field is sensed in the adjacent pipe through a “Hall effect sensor”(no relation to the present inventors). The Hall effect sensor produces an electrical signal corresponding to the magnetic field strength and sends the signal along a conductor wire to the coil and the next joint.
Although U.S. Pat. Nos. 4,806,928 and 4,901,069 to Veneruso do not describe a system that is incorporated into individual sections of drill pipe; these patents do show a system for electromagnetic coupling a cable passing through the well bore to a downhole tool. The system described includes inner and outer induction coils which are cooperatively arranged and adapted so that one of coils can be dependently suspended from a well bore cable and lowered into coaxial alignment with the other coil that is positioned within the well bore and electrically connected to a downhole apparatus.
Another example of a downhole data transmission system that uses the principles of induction is described in U.S. Pat. No. 4,605,268 to Meador. This patent shows a current-coupled system that uses two toroidal coils at each joint. Each coil is confined within an electrically conducting housing. A first electrically conducting housing surrounding the first coil, located in the end of one drill string component, is electrically connected to a second electrically conducting housing for the second coil, located in the end of the adjacent drill string component. In this way, as an electrical current is induced by the first coil in the first electrically conducting housing, that electrical current is conducted to the second electrically conducting housing, whereupon, a magnetic field is induced in the second coil. Thus, although the principles of induction are used, the system in the “268 patent relies on an electrical connection between adjacent components of the drill string. As such, it is subject to the problems described above in connection with the third category of systems.